不確定~好久沒碰微積分了~

用你的假設L1,L2.. , @ -> theta .
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sin @ = L2/8
cos @ = L1/1

L1 + L2 = 8sin@ + cos@
L = 8sin@ + cos@
L' = 8cos@ - sin@ = 0 ==> 8cos@ = sin@ ==> 8 = sin@/cos@ => 8 = tan@ ==> @= atan(8)-->解出來@ ..
帶回L= ... 即可求出極值.
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沒仔細算過..也許是錯的喔 XD~